Electrometers

Equation 40 states that the pH sensitive, galvanic cell potential (E) to be amplified is 

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Usually one varies the head of mercury or applied gas pressure so as to bring the meniscus to a fixed reference point [118], Grahame and co-workers [119], Hansen and co-workers [120] (see also Ref. 121), and Hills and Payne [122] have given more or less elaborate descriptions of the capillary electrometer apparatus. Nowadays, the capillary electrometer is customarily used in conjunction with capacitance measurements (see below). Vos and Vos [111] describe the use of sessile drop profiles (Section II-7B) for interfacial tension measurements, thus avoiding an assumption as to the solution-Hg-glass contact angle. 

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

The greatest potential appHcation for single-electron devices Hes in digital circuits. However, a number of other appHcations exist, including current standards and ultrasensitive electrometers (70,71). SETs are not unique to compound semiconductors, and in fact a great deal of work has been carried out in other material systems, including Al—AlO —A1 tunnel junctions. A review of single-electron phenomena is available (72). 

Tests Simulating Use Conditions. Several simulation tests involving wear and use testing have been developed. The first and only one to be adopted as a standard in the United States is AATCC Test Method 134-1975 for measuring the electrostatic propensity of carpets (60). In this test a person wearing standard neoprene and leather sole shoes and carrying a probe connected to an electrometer walks on a carpet for 30 to 60 s until the measured body voltage builds up to a maximum. 

Of the instmments described only some are suitable for use outside the laboratory. Where any instrument is carried into a flammable environment it should either be certified as intrinsically safe for exposure to the flammable atmosphere or isolated from the atmosphere such as by keeping it within a purged enclosure. Any probe connected to the instmment must be separately considered as a possible ignition source. Electrometers are described in [ 1531. A more general review of electrostatic instruments is given in [ 136]. 

The input impedance of electrometers is of the order 10 2. Electrometers may be equipped with either capacitance or resistance voltage dividers to enable their voltmeter input range (typically 10 to 100 V) to be increased to 30 kV or more. The capacitance divider comprises an isolated target elec-... 

By using a flame ionization detector (FID), most compounds having a bond of carbon and hydrogen can be measured. This detector was originally developed for gas chromatography and employs a sensitive electrometer that measures the change in ion intensity resulting from the combustion of air... 

Blatt-eisen, n. sheet iron, -elektrometer, n. leaf electrometer. 

Viewing things from the perspective of his physical theory of contact electricity, Volta was intrigued by the apparently endless power of the battery to keep the electric fluid in motion without the mechanical actions needed to operate the classical, friction, electrostatic machine, and the electrophorus. He called his batteiy alternately the artificial electric organ, in homage to the torpedo fish that had supplied the idea, and the electromotive apparatus, alluding to the perpetual motion (his words) of the electric fluid achieved by the machine. To explain that motion Volta relied, rather than on the concepts of energy available around 1800, on his own notion of electric tension. He occasionally defined tension as the effort each point of an electrified body makes to get rid of its electricity but above all he confidently and consistently measured it with the electrometer. 

Potential measurements—a reference electrode and a potentiometer or electrometer which require only a small current to give a measurement of e.m.f. and thus minimise polarisation of the electrodes. 

The preferred potential-measuring instruments are potentiometers or electrometers, either of which permit measurements to be made without flow of sufficient current to polarise the electrodes during the determinations. It is also possible to use millivoltmeters if the internal resistance of the instrument is high enough to avoid any appreciable flow of current. 

Fig. 20.4 Lippmann electrometer for studying the variation of the excess charge on mercury with variation in potential difference at the mercury solution interface...

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